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http://www.necrohost.com. Provides a list of URLs that simulate a variety of network connectivity issues. Slow response, unresolvable dns, 404 and several other tools.
Another useful site for the toolbox: http://www.viewdns.info/ - A collection of free, useful DNS tools.
I've found Fiddler to be invaluable for web development.
A load testing service should always be part of your toolbox, not just checking page weight, (this may be biased, I'm the founder), so would also recommend adding something like Loadzen
I got another one of the 'everyday' category: free private repository hosting!
While github is nice, it's everything but free for your private source control hosting needs. Luckily there is:
+1 for Fiddler.
For multi browser screenshots I find https://browsershots.org/ to be brilliant.
There is also http://isitup.org for the 'monitoring' category.
+1 for jsFiddle, cant live without it
Some from me:
You could try http://www.nuvolabase.com/, a OrientDB based HTTP/REST powerful database on the cloud
Similar to isitup.com and PingDom, is my project http://www.pingbrigade.com.
I like http://jsbin.com/ more than http://jsfiddle.net/
Reset all css to zero before doing your own: http://meyerweb.com/eric/tools/css/reset/
960 grid system: http://960.gs/ - Generator: http://grids.heroku.com/
And my personal favorite app, not only for documentation:
There's actually a similar "live list" of web development tools. It's called The Toolbox, made by Sacha Greif. If anyone is interested, the source code is on GitHub.
Thanks for mentioning The Toolbox! But I should mention its source code is just the WP source code, there's nothing special there (and it's not open source, sorry…).
Surprised nobody's mentioned http://JSLint.com, or are you all past it? Also, http://web-sniffer.net helped me figure out that my server wasn't doing the right thing with .manifest files when I started messing around with offline iPhone apps.
http://jsonlint.com - to check that you are getting or returning valid JSON
A couple of projects I have:
Hey everyone -- thanks for all the excellent suggestions!
I've updated the list with several new tools, most of which you (or the people on HN and reddit) have suggested:
Some of the suggested tools that were suggested but didn't make the list are:
Also, thanks @iignatov for a pointer to The Toolbox, that site looks really awesome! I just wanted to create a list focused more on Web engineering (protocols 'n' stuff), less on Web site design.
Thanks again, everyone! Let me know if you come across any new tools.
For the last section I would suggest www.browserleaks.com site with many interestings. Also there is a vim/emacs-killer of the century - www.sublimetext.com, it replaces me lot of things from your list.